1. Field of the Invention
This invention relates to an incremental measuring system, particularly a length measuring system, comprising a scale member provided with a scale which is adapted to be scanned preferably by an optoelectronic scanning method, a scanning unit provided with scanning sets for scanning said scale and for generating measured-value signals in response to the scanning of the scale, and an evaluating unit for deriving digital countable signals from said measured-value signals, wherein said scale member is also provided with a reference track that is parallel to said scale and comprises a plurality of reference marks associated with respective reference points at fixed locations on said scale member, the scanning unit comprises reference mark scanning sets for scanning said reference marks, said scale member is also provided with a control track and a reference signal delivering circuit is provided, which under the control of said control track delivers reference pulses only in response to the scanning of selectable ones of said reference marks.
2. Description of the Prior Art
Such measuring systems are known, e.g., from Published German Application No. 34 45 694 and permit an optoelectronic scanning of the scale by the scanning unit preferably in a non-contacting manner. Other known measuring systems comprise scales which can be scanned by inductive or magnetic or capacitive methods and are provided with corresponding reference marks. The measured-value signals which are generated in response to the scanning of the scale are periodic analog signals, which in most cases are basically sinusoidal and which comprise a complete signal period for each complete increment of the scale, which increment in case of an optoelectronic scanning consists of a bright field and a dark field of the scale. In the previous practice, two measured-value signals were generated, which in most cases were displaced 90.degree. in phase. In dependence on the direction of movement of the scanning unit, one of said signals or the other will lead the other and the direction of movement of the scanning unit is indicated by signals generated by a direction detector. The analog measured-value signals are processed to provide digital countable signals. The scale may be electronically or arithmetically subdivided, in most cases by a microcomputer. The digital countable signals may be used to control a machine or a robot and/or to display the result of the machine or a robot and/or to display the result of the measurement on a display unit. To establish a definite relationship between the countable signals and the locations at which the scale is being scanned, the counting means must be set to predetermined values, such as zero, when the scale member is being scanned at predetermined or preselectable locations. This is accomplished with the aid of reference signals, which are generated is response to the scanning of the reference track. When the counter is set to a defined value in response to the reference signal, that value will indicate the location that is associated with a selected zero point of the display to the associated reference mark. A plurality of reference marks may be provided and will permit a generation of reference pulse at selected locations in various length portions of the scale. In that case it will be necessary to identify the reference mark which is being approached so that the control or conversion can be related to a certain location on the scale member. Expensive control and switching systems have previously beend used to distinguish selected reference marks from non-selected ones. Alternatively, non-selected reference marks may be covered or removed although this is an expensive operation if the measuring system is encapsulated, as will mostly be the case, and can be performed only by skilled labor and reference marks which have been destroyed cannot be re-applied. Known selecting systems comprise mechanical on-off switches, which have been disclosed in Published German Application No. 18 14 785 and are actuated by separate stops during the movement of the scanning unit and which activate or enable the control circuit only in response to the scanning of pre-selected reference marks. In accordance with German Patent Publication No. 24 40 412 said mechanical switches have been replaced by magnets, which are attached to the scale member at preselected locations and serve to actuate selector switches, which are provided in the scanning unit and consist of reed relays. It is also known to provide the reference marks in a plurality of reference tracks and to associate a scanning unit with each track and to define a reference mark in that the scanning unit which is associated with a given track is enabled to select reference signals. In that case too a change of the reference marks and the setting operation require a considerable expenditure of work, time and equipment and can be performed only by skilled labor. Whereas erasable reference marks may be attached at closely spaced locations, all reference marks which are not required at a given time must be mechanically removed or must be covered by stuck-on slips. That practice also involves the disadvantages mentioned above. For this reason it has been proposed in EP-A-0 239 768 to remove two reference marks which are disposed on opposite sides of and spaced equal distances from a selected reference mark and to scan the reference marks with two scanning sets having the same spacing as the reference marks and arranged in an antiparallel connection. In that case the reference signals produced by existing reference marks will eliminate each other and a usable reference signal will not be generated unless an existing reference mark and the region of a reference mark which has been removed are scanned at the same time. In that case the reference marks must be provided with an exactly uniform spacing and the scanning sets must not depart even by an extremely small distance from a desired position relative to the scale and to the reference track. In that case too a subsequent change of the selected reference marks will either involve a high expenditure or will be impossible.